5 Stars worth watching


Alpha Centauri, the Sun's nearest neighboring star system, seen by the Cassini orbiter above the limb of Saturn. / NASA/JPL/Space Science Institute

Sometimes I worry that the popularity of cosmology—the study of the universe on the very largest and smallest of scales—has created a significant misconception about astronomy among the public. Specifically, in cosmology, the perceived worthiness of a question seems roughly proportional to the increasing number of light-years that separate us from its answer. In this view, most events considered interesting will occur a trillion years from now, or already took place many billions of years ago on the other side of the universe. This is exactly opposite from much of the rest of observational astronomy, where many of the most treasured objects are those that, because they are relatively nearby, offer plentiful photons for astronomers to gather and study.

This is particularly true in the search for habitable exoplanets. Planet-hunters need lots of photons from a star to confidently determine whether it harbors any planets, and they need even more photons from a planet to have any notion of whether it harbors anything alive. The relatively low number of available photons from the distant stars in the Kepler field is what will keep us from closely examining more than a handful of the many transiting, potentially habitable worlds we will glimpse there. And it's the pursuit of more photons that drives the costs and specifications of ambitious space telescopes that, once built, should be able to study the atmospheres of small, rocky worlds like ours.

What this means, and what I worry many people don't realize, is that we probably don't need to gaze across the universe or even the galaxy to find another Earth-like planet. In fact, we may only need to look right next door. It could be that, to have a good chance of gathering enough photons to find life beyond our solar system, we need to only build a very large and expensive space telescope for perhaps $5-10 billion, rather than a ridiculously large space telescope that would be an order of magnitude more expensive.

I recently wrote a feature story for Nature exploring how focusing on nearby stars could help reduce the astronomical projected costs of finding and verifying living worlds beyond our solar system, and I'm planning to talk more about the field's near- and long-term prospects later here on BoingBoing, but for now, here's a quick listing of five relatively nearby stars that could soon yield a high-value, world-changing discovery, and kickstart serious efforts to make the next giant leap in observational astronomy.

Pay attention—before you know it, one of these stars may be making front-page news.

1. Alpha Centauri

I'm cheating a bit here, because Alpha Centauri is actually two Sun-like stars, Alpha Centauri A and Alpha Centauri B, orbiting each other with an average separation of a bit more than the distance between our Sun and Uranus. There's also a red dwarf, Proxima Centauri, drifting in the outskirts of the system. Both A and B have stable regions in their habitable zones where small planets could exist, though there is some debate over how easily planets can form around such binary pairs. Most importantly, at only 4.3 or so light-years away, the stars of Alpha Centauri are the closest in the sky other than our own Sun. They're very bright, and very easy to study, though no planets have yet been found. At least three planet-hunting teams are currently performing radial-velocity surveys of Alpha Centauri, with a particular focus on B, which is a more quiescent star, and thus easier to monitor. For additional context about these stars and two of the competing teams, check out my 2009 feature story in Seed magazine, The Long Shot.

2. Gliese 581

This red dwarf star is relatively close by, just over 20 light-years away, and is one of the few stars of this very promising type to have been closely scrutinized for planets. Searches have discovered 4 planets and 2 candidates that await confirmation. Two of these objects, Gliese 581 d and the to-be-confirmed Gliese 581 g, are small enough and close enough to the star that they could conceivably be habitable. I wrote about Gliese 581 g, the so-called "Goldilocks" world, shortly after its discovery announcement last year, describing what its environment might be like. Since then, doubts about g's existence have received lots of media coverage, but one aspect of this story has gone largely overlooked: Even if many of the details of Gliese 581 g as reported in its discovery paper are proved incorrect, that particular orbital slot could well be occupied by another, smaller, potentially more habitable planet that has yet to be indisputably detected. Given the public uproar and resulting increased observational cadence for this system, I'm guessing we'll know soon whether Gliese 581 has a Goldilocks.

3. GJ 1214

At 40 light-years away, this red dwarf star is close to the cusp of my comfort-zone in labeling it "nearby." And I'm also being a bit generous in its potential to deliver a game-changing discovery for habitable planets. The only known planet this star does have, GJ 1214 b, is a warm "super-Earth" of about 6 Earth-masses, and isn't really very Earth-like at all. But GJ 1214 b is very special, because it transits—which allows astronomers to examine its atmosphere and learn more about the planet's composition and history.
Deeper investigation of worlds like GJ 1214 b is very important, because while Kepler and other surveys are finding them seemingly everywhere they look, no planets like this exist in our own solar system. Consequently, at present it's quite difficult for astronomers to say whether planets of several Earth-masses could realistically be habitable. These worlds may almost always turn out to be more like "mini-Neptunes" than super-Earths. GJ 1214 b is the most easily-studied planet of this sort known to us, but hopefully more examples will be found soon transiting nearby stars.

4. HD 40307

HD 40307 is a K star slightly smaller and cooler than our Sun. It's even further away than GJ 1214, lying 42 light-years distant in the constellation Pictor, but it's of great interest to planet-hunters because of its confirmed planets, three worlds discovered in 2008 in scorching, nearly circular orbits, all less than ten times Earth's mass.
These worlds were discovered via the radial-velocity technique, meaning that astronomers can only estimate their mass and don't have reliable information on their density or composition. Just as with most other super-Earths, it's not entirely clear whether they're large rocky planets with thin atmospheres or bloated balls of gas with smaller rocky cores. Given the growing number of known planets of this kind, this system doesn't at first glance appear very notable. But there are tantalizing residual motions in its radial-velocity signals that hint at an additional orbiting companion further out in the system, perhaps even in the star's habitable zone. The European Southern Observatory's HARPS spectrometer, the world's premier radial-velocity instrument, is closely monitoring HD 40307 in hopes of finding additional, more habitable planets there.

5. 61 Virginis

61 Virginis is a star very similar to our Sun, though probably a billion or so years older, just less than 28 light-years away. Like HD 40307, it's known to harbor at least three planets in scorching, tightly packed orbits, a super-Earth and two Neptune-sized worlds that were discovered in 2009. The system also appears to have a debris disk of cold dust in its outer reaches, likely the product of cometary impacts in an analog of our own solar system's Kuiper belt. According to calculations by members of the discovery team at the University of California's Lick Observatory, stable orbits exist in 61 Virginis' habitable zone. 61 Virginis is one of the top targets for Lick Observatory's new Automated Planet Finder Telescope, a facility dedicated exclusively to radial-velocity planet-hunting that is beginning operations this year. By having the ability to observe this and other target stars each and every night instead of in infrequent chunks, the APF could rapidly discover many small, rocky planets.

Got questions about why I chose these stars instead of others? Do you have your own preferred list of most promising places to look for potentially habitable planets? Let me know in the comments!


  1. It is a shame that Carl Sagan is not around. This is all so incredibly exciting and so much of my interest in this was fueled by him that when I read about exo-planetology or life elsewhere I think of him.

  2. VERY interesting article. These are close enough that an optical scope in orbit will likely pick up disks of any planets that are there. My interest is how the angular momentum data matches up with what is observed, to give a confirmation of what we think is working with suns that are further away.

  3. How come if we can see Alpha Centauri as a double star system there aren’t any other stars showing up in the photo? NOT trying to be a conspiracist or anything, just curious.

    1. Both Alpha Centauri A and B are some of the brightest stars in the sky, because they’re relatively large and very close to us. The only stars that outshine them in our skies, other than the Sun, are all *huge* stars, like Canopus, Sirius, and Arcturus.

      So it doesn’t really take a long exposure to get a good picture of them. I’m guessing the exposure was too short for other, fainter stars to show up.

      1. Sirius isn’t that big as stars go, and it’s not that far away – less than three times the distance to Alpha Centauri.

    2. @Tatsuma – I can see two stars in that photo. They’re pretty close together but there are definitely two bright objects sitting on the edge of the ring.

  4. Great post (again!).

    I do have to say: it might seem in astronomy circles like cosmology is taking up the limelight. From my view outside though, cosmology is a much-needed promotion of the value of considering the stars, and is entirely complimentary to astronomy. In other words, more consideration of cosmology brings more consideration of astronomy, from what I see.

    And almost anything that gets people thinking of something larger than the latest celebrity news is a good thing in my book.

    1. I agree for the most part. Cosmology asks lots of deeply fascinating and profoundly important questions, and knowing their answers gives us a better sense of our place in the universe, just like other subdisciplines of astronomy/astrophysics.

      But given the extremity of scale that cosmology operates on, “our place in the universe” shrinks to a rather indistinguishable dot. What I like about the search for habitable planets is that it expands that dot of familiarity outward into our entire galaxy and beyond. The stars would perhaps seem less cold and distant if we knew we weren’t alone. And that’s worth considering when it comes to the necessary evil of prioritizing funding.

  5. From reading the abstract from the Cornell library link, is the probability of finding a potentially habitable planet really 10%? That seems like an incredibly high value.

  6. Good post, as usual!

    As for stars I’d add to the list; I’ve always had a good feeling about Eta Casseiopeiae (sure, it’s a binary system, but still)… dunno why.

  7. One of my old professors has worked on a starshade design for a space-based telescope that could resolve significant details in such a search of nearby star systems. It’ll likely never see funding, but it, and the many other designs for both imaging and spectroscopy show that there are plenty of opportunities for doing good science even if we can’t investigate directly.

  8. Aside from Alpha Centauri (which I remember from my amateur astronomy days), are any of the stars on the list visible to the naked eye?

    I’d love to be able to go out in my backyard and look at a star that we know has a planet around it. That’s something which would fire my imagination and maybe rekindle my long-dormant interest in astronomy.

    Until that point though, yeah, cosmology is way more interesting to me and I absolutely see why it gets more attention.

    1. Hi Anon,

      Lots of known planet-bearing stars are visible the the naked eye! And, if planets are as prevalent as I think they are, when you look up at the night sky and see a star, chances are there’s something “planetary” around it, albeit perhaps only quite small. But back to the ones we know about for sure… a preponderance of them are most easily seen from the southern hemisphere, it seems, because that’s where a lot of the best observatory sites and equipment are located.

    2. Here are some nice bright stars that have known planets, and are easy to find:
      1. Fomalhaut
      2. Pollux
      3. Gamma Cephei
      4. Gamma Leonis
      5. Epsilon Tauri
      These are all giant planets, not the little habitable worlds Lee is excited about. But I’d recommend them if you just want to see another system with your own eyes.

  9. Great post. I think a focus on seeing the details of nearby planets would be a great thing, even if we don’t discover a habitable one. We live in a universe full of planets and moons. It seems that there’s a huge amount of variety in what these places can be like, but we have only a limited number of examples in our Solar system. The more specifics we can find out about the worlds nearby, the better we can imagine what the rest of the universe is like.

    One thing I’d really love to see is spectroscopy data from exoplanets that would allow detailed analysis of their atmospheres’ composition – I’m certain that kind of data would throw up many fascinating surprises and would give us a far better idea of what planets in general are like. I don’t know how hard it is to get that kind of data, but I bet it’s much easier if the planet is nearby.

    In particular, as was originally pointed out by James Lovelock, the atmosphere of the Earth with its biosphere is far from chemical equilibrium, unlike the other worlds in the Solar system. If we could detect that another planet’s atmosphere was far from equilibrium we would know that something very interesting is going on, even if the planet is not in the region generally recognised as “habitable” by life-as-we-know-it. It would suggest (though not prove) that life or something like it can exist in a form chemically very different to that of Earth. To me that’s the kind of surprise we should be looking for.

  10. Alpha Centauri.
    Funny you should throw that one in the list. My own understanding is that if there are planets they’d be small “terrestrial” worlds sticking close to their parent star. Anything 5 AU and out would be subject to the tug from the companion star and possibly ejected. As much as I’d love to see a stable planetary system there, I have my doubts.
    But I really want to be proven wrong on that.

    1. Yep, you’re right. But from an Earth-like/habitability perspective, we aren’t really that interested in anything orbiting several AU out from either Alpha Centauri A or B. We’re interested in the closer-in terrestrial planets that might be there.

      I talk about this a lot more in the 2009 feature story (http://seedmagazine.com/content/print/the_long_shot/). One of the big sticking points is that, while stable orbits do exist in the habitable zone, it’s unclear how exactly planet formation would proceed in the first place around this binary-star pair. So there is a chance that Alpha Centauri is orbited only by sand and dust.

  11. No argument with the first three. What about replacing the last two, why not :

    Epsilon Eridani, the closest star with a confirmed planet ? It’s only 10ly away. Anything that close just feels like it should generate important information.

    Or Gliese 876, with four planets, and about 15ly away ? The discoveries keep popping up on this one, the longer it’s studied.

    1. Plus Epsilon Eridani has two asteroid belts and a thick outer disk, and based on them probably has at least two more planets we haven’t confirmed.

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